This invention relates to same-frequency repeaters.
A same-frequency repeater (SFR) is a device that receives a radio broadcast signal at a particular frequency and at a relatively low power level and rebroadcasts the received signal at the same or nearly the same frequency but at a much higher power level. This is in contrast to repeaters that are in more common use in which the signal is typically rebroadcast at a frequency different enough from the received frequency so that there is essentially no spectral overlap. The term single-frequency repeater is also used. The use of such different frequencies makes it relatively easy to effect electrical isolation of the high-power transmitted signal from the input stages that are adapted to receive low-power signals. The principal cost of this advantage is that twice the spectrum is required for a system that operates at two frequencies. The SFR minimizes the amount of spectrum used but creates the problem of spillover. This is the term applied to that portion of the transmitted signal that is received in the input to the SFR. Spillover is also referred to as leakage or blowover. Since the amplification in a repeater may easily reach 160 or 170 db, the spillover that is detected at the receiving antenna may be down many db from the transmitted signal and still interfere with the received signal.
The presence of spillover within 10 dB of the desired input signal to an SFR may lead to several bad results. There will, of necessity, be a time delay in processing a signal through an SFR. Such a time delay can be of the order of 100 microseconds. If the signal is voice-modulated and both the signal and its delayed spillover are rebroadcast, the resulting signal is greatly distorted. If the signal carries digital data, a delay of more than one-third of a bit width loses intelligibility. This is about 30 microseconds for a bit rate of 12 KHz. If the spillover frequency is received at a sufficiently large magnitude, there is the possiblity of oscillation in the repeater. If the spillover is significantly stronger than the received signal, the received signal will be lost. It is evident that if the spectrum conservation of SFR is to be achieved, spillover from the output into the input must be controlled.
One known way to control spillover is to combine a tag signal with the transmitted signal so that the spillover is tagged. The use of a feedback system to minimize detected tag in the input can control the amplitude and phase of a sample of the transmitted signal so that its addition to the input cancels spillover. Such control is difficult to implement for broadcast frequencies of 40 MHz and higher.
It is an object of the present invention to provide a method and means of controlling spillover in an SFR.
It is a further object of the present invention to apply a double-sideband suppressed-carrier signal as a tag on the broadcast signal of an SFR.
It is a further object of the present invention to effect cancellation of spillover in an SFR by minimizing detected tag signal in the IF frequency.
Other objects will become apparent in the course of a detailed description of the invention.